1. Field of the Invention
This invention relates generally to a method for achieving the required intimate contact of metallic detail parts to be joined by solid state diffusion bonding by properly shaping the detail parts to permit application of sufficient force to plastic flow the material into the required intimate contact.
2. Prior Art
Aerospace power conversion equipment which utilizes the change in velocity and direction of gas flow requires the use of rotating blades and stationary vanes. For airborne equipment it is necessary that sturctural concepts, yielding maximum strength and stiffness to weight ratios, be utilized to achieve the required power to weight efficiency. Blades and vanes are thin aerodynamic shapes with varying degrees of camber, twist, and thickness, as a function of the gas flow requirements. The gas pressure and flow characteristics impose bending strength and stiffness requirements upon the structural configuration of the blade or vane.
It is known that the lightest weight structures are achieved by utilizing a material having a high strength to weight ratio in conjunction with a structural configuration which places this material at the periphery of the structure, i.e., a hollow section. However, when using a hollow section with thin material, the material becomes unstable in compression and shear buckling modes. The use of internal stiffeners to stabilize the thin facing skins of hollow sections has been developed and proved in service for many years. While such internally stiffened shell structures are known in the prior art, they have been used in substantially planar configurations and large shells, and heretofore it has not been known how to fabricate them in the intricate shapes and with geometric forms required by many applications, such as, for example, the relatively small aerodynamic blades and vanes. Thus, the present invention now makes available for applications requiring contoured structures, the significant advantages of lightweight, structurally efficient, thin skin shell structures with internal stiffening material. Such lightweight structurally efficient shell structures are highly advantageous when used in applications such as, for example, power conversion equipment, helicopter blades and turbo engine fan blades and vanes. In addition, the present invention achieves its high structural strength to weight ratio within the economic constraints of system cost effectiveness whether used for flat sandwich panels or shell structures for such applications as aerodynamic blades and vanes.
The most common method today for fabricating aerodynamic blades and vanes is by forging solid blanks, followed by 100% machining to achieve the desired shape and contours. While net precision forgings may also be produced, these require use of special alloys known in the art, but the latter are not as efficient as the higher strength wrought alloys. In the case of larger vanes, built-up brazed assemblies are typically produced. Each of these present methods are relatively costly and produce structures which are heavier than desirable. Recent "advances" in the art, such as the filling of hollow structures with suitable potting compounds, have enabled the production of somewhat lighter vanes. However, the vanes so produced have suffered from a disappointingly high failure rate because of the strain incompatability of dissimilar materials. A further shortcoming of blades and vanes produced by the methods of the prior art, other than solid structures, are their susceptibility to catastrophic failure caused by foreign object damages.
The invention disclosed in my earlier filed patent application, Ser. No. 357,359, taught one method utilizing prefabricated panel material to overcome these shortcomings and limitations of the prior art. The present invention represents a further advancement in the method of fabricating shell structures having extremely lightweight and great structural integrity. By the use of a specifically designed self-stable internal sheet metal stiffener, the required intimate surface contact for diffusion bonding can be achieved by plastic flowing the detail stiffener or stiffeners into the required contact simultaneously with the diffusion bond joining of the leading and trailing edges. The present invention thus lowers costs, yet produces a unique shell structure design which is extremely strong.
The phrase "structural integrity", as used herein, relates to the strength and stiffness of the structure per pound, and its resistance to catastrophic failure from foreign object damage. High structural integrity is attributable to the parent-material homogeneity achieved by the diffusion bonded joining techniques disclosed herein. The value of this invention is best illustrated by reference to turbofan engine fan blades. As indicated above, these blades are now typically machined from solid material. Hollow, internally stiffened shell structured blades made of the same material by the present invention would have approximately one-third of the weight of aforedescribed solid blades. Weight saved on a rotating fan blade results in additional weight and economic savings in the full engine configuration by virtue of the reduction of the loads on the fan disc, main shaft, bearings, support structures and containment shrouding. The weight saving multiplier in a typical turbofan engine is in the range of 3-5. Thus, for each pound of weight saved on a fan blade, 3 to 5 pounds of weight can be saved in the totally configured engine.
The present techniques for producing homogeneous material structural components include machining from bar or plate stock, net forging, forging plus machining and extruding (for constant section members). These production techniques are not applicable to the fabrication of thin wall and hollow sections with internal stiffeners. In addition, the cost of machining aerodynamic thin shapes with various thickness, camber and twist from solid stock is very high.
Because of the limitations on producing one-piece homogeneous internally stiffened aerodynamic shell structures, techniques for joining components of the vane or blade together must be used. All typical production methods of joining metals, such as by riveting, bolting, welding, brazing, organic bonding and polyimide bonding result in a load transfer capability lower than that of the parent material utilized; i.e., they do not achieve the required homogeneous properties and strain rate of the parent material across the joint. Solid state diffusion bonding techniques as used in the present invention, on the other hand, provide (i) a means for achieving full parent material strength across the joint interface because no foreign material is utilized; and (ii) strain compatability across the joint interface. Several diffusion bonding techniques have been developed, such as, for example, roll bonding, press bonding and vacuum bag bonding. Each of these known techniques of diffusion bonding and their respective limitations and shortcomings are briefly described hereinbelow.
The roll diffusion bonding technique utilizes a steel tooling retort with positioning filler tooling to locate the members to be joined in proper respective positions. The intimate contact is established by roll reducing the retort/tooling and to-be-joined parts by a sufficient percentage (generally 50% to 60%) to guarantee completely intimate surface contact and diffusion bonding. This process utilizes expendable tooling and is basically limited to the attachment of members in the rolling direction. For complex shaped parts such as vanes or blades, this expendable tool is cost prohibitive.
The press diffusion bonding technique utilizes reusable positioning and restraining tooling and massive hydraulic presses as the pressure source to establish the intimate contact. However, to utilize reusable tooling, the local surface deformation is generally limited to less than 5%. This requires the surfaces to be joined to be matched within very close tolerances. In addition, it also requires, because of the relatively low local unit pressure, a long time at an elevated temperature to allow the diffusion cycle to complete. For complex shaped parts such as vanes or blades this close tolerance matching of detail parts is cost prohibitive.
The vacuum bag bonding technique utilizes atmospheric pressure as the pressure source and is therefore limited to very thin sheet structures which can attain the required intimate surface contact at this relatively low pressure, again with the requirement of detail parts matched within very close tolerance. Because of this low pressure, the time at an elevated temperature required to complete the diffusion cycle is very long.
As a result of the above-described limitations in the techniques of joining metals, prior art turbofan engine fan blades, for example, are either completely machined from wrought bar stock or forged and completely machined, notwithstanding the high cost of such methods and the lower strength to weight ratio of the resulting blade as compared to that attainable with hollow internally stiffened shell structures. The present invention, however, overcomes these limitations of the prior art and discloses a method of utilizing the plastic flow characteristics of metal at elevated temperature under pressure to achieve the intimate contact required for diffusion bonding. The unique self stable configuration of the internal stiffener disposed therein and diffusion bonded together presents a practical method for fabrication of complex shaped hollow internally stiffened parts such as vanes and blades.